"Ah, John," I replied, "you have forgotten that the earth is rushing along and carrying the moon with it nearly as fast as we are travelling, and you are reckoning as though they were standing still all the time. As a matter of fact we are only gaining on the moon by a little over fifteen thousand miles an hour, and we must allow for slackening speed long before we reach the moon, so we cannot expect to cover the distance in less than sixteen hours. You will see that if we did not travel faster than the moon is moving away from us we should never catch it up at all!" "That explains it all, Professor," said John, "and I must confess I felt rather puzzled at the length of time required to reach the moon, so was altogether out in my calculations." After we had been proceeding at this rate for nearly two hours, M'Allister came hurrying into our compartment in a state of great excitement. "Professor," he exclaimed with a gasp, "something's gone wrong altogether, and I don't know what to do!" "Gone wrong!" I repeated. "Why, what is the matter?" "Mon," he answered, "everything is the matter! A while back we were rushing towards the moon, but just now when I looked ahead there wasn't any moon to be seen. I happened to go round to the other window and look back and, my word! if there wasn't the moon right behind us! We have been travelling so very fast that we must have run past it without knowing we had done so." "Oh, we could not possibly have done that!" I exclaimed. "But there's more to come, Professor," continued M'Allister. "When I last saw the moon it was nearly full and not so very much bigger than when we saw it at starting, but now this moon behind us is an enormous thing; yet it is only a new moon, or rather what folks call a new moon with the old moon in its arms!" "Oh, now I understand," I replied. "It's all right, M'Allister, and you can make your mind quite easy. You were not able to see the moon when you first looked through the window because it was nearly in a direct line with your course, and therefore just hidden by the prow of the vessel. It's still ahead of us and still nearly full: if you had looked out of the conning tower or used the periscope you would have seen it." "Heh, Professor," he interjected, "I know I couldn't see the moon if it was straight ahead of our course, but then what about that enormous new moon that's behind us? I saw that right enough." "That enormous new moon, M'Allister, is only our own little world which we left a few hours ago," I replied. He stared at me as though bewildered, and after pondering a while, exclaimed, "Losh, mon, you surely don't mean to say that our own little world changes about in the same way as the moon does--sometimes new and sometimes full?"